The inventive concepts described herein generally relate to oscillators, and more particularly, the invention concepts relate to digitally controlled oscillators.
Generally, a digitally controlled oscillator (DCO) must have a relatively wide tuning range in order to execute digital automatic frequency calibration of a phase locked loop (PLL) or a voltage controlled oscillator (VCO) of a wide operating range.
In the meantime, when frequency is controlled in a ring oscillator type DCO using a digital-to-analog converter (DAC) and a current starved transistor, a transistor used for a current source has a size that is significantly larger than a transistor used for a ring oscillator, and it is difficult to obtain a wide tuning range of about 2 decades.
In an effort to obtain an oscillator of a wide tuning range, a resistor or a capacitor may be switched according to a digital code. In this case, a frequency output from the oscillator varies generally along a 1/x curve since the frequency is inversely proportional to the resistance and the capacitance. If the frequency varies nonlinearly, the resolution of the frequency, which varies with the digital code, is degraded.